The present invention relates to a handover method for a CDMA mobile communication using a spread spectrum signal, for a base station and a mobile station of the same.
The CDMA method is a leading candidate for the next generation of mobile communication systems as a multiple access method capable of rapidly increasing the system capacity (number of channels). Progress is being made in standardization for practical use.
Presently, in the wideband CDMA mobile communication system being standardized, communication information between a base station and a mobile station is spread in frame units. The frame units are obtained by partitioning the information into predetermined lengths, for example, 10 msec (hereinafter, this frame unit will be referred to as a “radio frame”).
An upstream traffic channel signal from a mobile station is spread using a spread code initialized for every “super frame” comprised of 72 radio frames as a unit period. For this reason, when performing control to switch (handover) a communication channel from a first base station in communication to a second base station when a mobile station moves from a radio zone of the first base station to a radio zone of the second base station, it is necessary to synchronize the upstream traffic channel signal in communication with the phase of the spread code at the second base station for reception of the upstream traffic channel.
In other words, it is necessary to correct the discrepancy of frame numbers (corresponding to the phases of the spread code) of the radio frame between the first base station in communication and the second base station. Further, it is necessary to correct the discrepancy of frame timings within one frame time. For this reason, a perch channel signal giving reference timing for measuring the discrepancy of the timings of the radio frames is transmitted from the second base station. The mobile station measures the discrepancy of the radio frames between the first base station in communication and the second base station based on this perch channel signal, and then sends the information to the first base station in communication. The second base station receives the information via a higher rank communication network and corrects the radio frame number for receiving the upstream traffic channel signal and the timing thereof, that is, the phase of the radio frame, according to the information.
FIG. 16 shows the phase correction of the radio frame of the related art. (A) of the figure shows a transmission radio frame for a downstream traffic channel signal of a first base station BS1 in communication. (B) of the figure shows a received radio frame of the downstream traffic channel signal of a mobile station MS before handover. (C) of the figure shows a transmission radio frame of the upstream traffic channel signal of the mobile station MS.
(D) of the figure shows the transmission radio frame of the perch channel signal of a second base station BS2. (E) of the figure shows the reception radio frame of the perch channel signal of the mobile station MS and (F) of the figure shows the reception radio frame of the upstream traffic channel signal of the second base station BS2.
Further, (G) of the figure shows the transmission radio frame of the downstream traffic channel signal of the second base station BS2. (H) of the figure shows the receiving frame of the downstream traffic channel signal of the mobile station MS after handover.
The transmission radio frame (A) of the downstream traffic channel signal from the first base station BS1 is received at the mobile station MS along with a propagation delay of the radio section. The mobile station transmits the radio frame (C) in the upstream traffic channel after an elapse of a predetermined timing (1024 chips) from the reception radio frame (B) of the downstream traffic channel signal.
At the time of the handover, the mobile station MS receives (E) the perch channel signal transmitted (D) from the second base station BS2. The mobile station then extracts the frame number written in each radio frame of the perch channel and, measures the discrepancy between that frame number and the frame number of the upstream traffic channel presently being transmitted, and the discrepancy of the timings of the radio frames within one frame. The mobile station MS sends radio frame phase discrepancy information TDHO indicating the discrepancy of these frame numbers and the discrepancy of the timings of the radio frames within one frame to the first base station.
The first base station BS1 sends the radio frame phase discrepancy information TDHO to the second base station BS2 via the higher rank communication network. The second base station BS2 then corrects the reception phase of the radio frame in the upstream traffic channel based on the radio frame phase discrepancy information TDHO and brings the phase of the reception radio frame (E) of the upstream traffic channel signal of the second base station BS2 into coincidence with the phase of the transmission radio frame (C) of the upstream traffic channel signal from the mobile station MS. Note that the phase correction of the propagation delay in the radio section is also carried out.
The second base station BS2 transmits (G) the downstream traffic channel signal with a timing advanced from the phase of the reception radio frame (F) of the upstream traffic channel signal by 1024 chips. The reason for this is that, as mentioned above, the mobile station transmits the radio frame in the upstream traffic channel after the timing of 1024 chips from the phase of the reception radio frame (H) of the downstream traffic channel signal, so as to coincide with that timing.
In other words, in the phase correction of a radio frame of the related art, the mobile station MS must extract the frame number written in the radio frame in the perch channel and measure the discrepancy of the timings of the radio frames within one frame between the perch channel and the upstream traffic channel.
In view of the above discussion, when frequency bands used between the base stations differ, it is necessary that a mobile station receive a downstream traffic channel signal from the base station in communication at the time of handover. It is also necessary that the mobile station receives a perch channel signal transmitted by the second base station while despreading and decoding the downstream traffic channel. Further, the perch channel must also be despread and decoded to extract the frame number thereof. Thus, the functional circuits of a mobile station have become complex. However, since a mobile station should have a minimal size and weight, it would be highly desirable to reduce the complexity of the functional circuits.